1. Field of the Invention
The present invention relates to a light emitting device including a light emitting element array on which plural light emitting elements are arranged, and emitting light toward a photosensitive material moving relative to the light emitting element array.
2. Description Related to the Prior Art
An optical fixing unit for a thermal printer is known as the light emitting device to emit light toward the photosensitive material. The optical fixing unit emits fixing light toward a thermosensitive recording paper, on which an image is thermally recorded by a thermal head, to optically fix thermosensitive coloring layers. The optical fixing unit is disposed on a conveyer passage, and emits the fixing light toward a whole recording surface of the thermosensitive recording paper on passage in a sub scanning direction. To optically fix the whole recording surface evenly, distribution of light quantity of the optical fixing unit should be uniformed in a scanning direction (a width direction of the recording paper).
As disclosed in Laid-Open Japanese Patent Application 2003-145812, a light emitting element array on which plural light emitting elements (for example LEDs) are arranged is known as a light source of the optical fixing unit. To uniform the light quantity distribution of the light emitting element array, it is ideal to arrange the light emitting elements with no clearance between each of elements. However, in practice, there is a need to make a clearance between the adjacent light emitting elements, for connection between wiring patterns of a circuit board and the each light emitting element, and so on. An uneven light quantity distribution occurs by these clearances. The unevenness of the light quantity distribution is compensated by diffusion of the light when a clearance between a luminescent surface of the light emitting element array and the recording surface of the thermosensitive recording paper is widened. However, if the clearance becomes wider, illumination efficiency and energy efficiency become worse, and illumination intensity is further reduced by the diffusion of the light. According to this, longer time is required for the fixation.
In Laid-Open Japanese Patent Application 2003-145812, a light emitting element array 100 as shown in FIG. 6 is described. In the light emitting element array 100, light emitting elements 102, 104 having width of about 1 mm are arranged to form a first element line and a second element line along the scanning direction, respectively. Each light emitting element is arranged in a zigzag form, such that the each light emitting element 104 in the second element line is disposed at a position corresponds to each clearance N1 (about 0.5 mm), which is formed between the adjacent light emitting elements 102 in the first element line, in the scanning direction. According to this arrangement, an experiment shows that even if a clearance L between the luminescent surface of a light emitting element array 100 and a recording surface of the thermosensitive recording paper 106 became about 20 mm, even distribution of light quantity in the scanning direction was maintained.
If the clearance L becomes narrower, the illumination efficiency and the energy efficiency become more enhanced. However, by the above arrangement, if the clearance L becomes narrower than 20 mm (for example about 10 mm), the uneven light quantity distribution occurs at positions where the light emitting elements 102 in the first element line and the light emitting elements 104 in the second element line overlap each other in the scanning direction.